Method of making a small electric motor

ABSTRACT

Concentricity between the rotor and rotor shaft of a small motor is attained by means of a plastic, toothed hub on the shaft press fitted into the rotor; any eccentricity is removed by shearing the teeth.

This is a division as of application Ser. No. 588,011, filed June 18,1975, now Pat. No. 4,035,676.

This invention relates to subfractional dynamoelectric machines, andparticularly subfractional electric motors.

Some subfractional electric motors embody a permanent magnet rotor ofcylindrical form mounted on a shaft. The present invention relates tosuch motors. Small synchronous motors are an example. The magnet rotormay be composed of magnetizable barium ferrite, and is magnetized toafford a plurality of magnetic poles angularly displaced around itsperiphery.

The rotor and shaft are connected by a hub. The mass represented by thehub adds inertia which inherently retards attainment of running speed.One object of the present invention is to afford a hub of less mass,thereby reducing the inertia. Also, the shaft is usually extended beyondthe hub to provide a thrust element to dampen undesired shaft movement.The lengthened shaft imparts more inertia. A related object of theinvention is to construct the hub to incorporate an integral thrustelement of light weight.

A problem also exists in connection with manufacture, namely, tomaintain concentricity between the shaft and the periphery of the rotormagnet. The critical dimension in terms of concentricity is to have theouter diameter (O.D.) of the supporting shaft concentric to the O.D. ofthe magnetic rotor. If this can be attained, a slight amount ofeccentricity between the shaft and the inner diameter of the rotor canbe tolerated. However, lack of concentricity in the critical dimension,the O.D. of the magnet, results in wobble. This, in turn, results inlateral thrust on the shaft bearing, wearing the bearing andcontributing to noisy operation. The larger the mass of the hubconnecting the rotor and shaft, the more these disadvantages areexacerbated.

Assured concentricity can be attained by accurately grinding both theI.D. and O.D. of the rotor magnet. This approach may be used with apre-formed continuous-rim hub either press fit or cemented to the shaftand to the rotor, as in U.S. Pat. No. 3,633,055. This is expensive. Themore common approach is to form the hub in situ inside one end of themagnet. This is usually accomplished by pouring or injecting a resin ina fluid state to fill one end of the magnet. When the resin hashardened, the hub is machined at its center in some fashion to presentan opening for the shaft, centered by means of a fixture. The manner ofmachining the hub may vary.

This approach to attaining concentricity embodies several time consumingsteps. Therefore, another object of the invention is to reduce theassembly steps for attaining concentricity and to afford more dimensiontolerance in the parts being assembled without sacrificing precision inthe completed rator assembly.

In the drawings:

FIG. 1 is an elevation of a subfractional synchronous electric motor inwhich the present invention may be embodied;

FIG. 2 is a sectional view of a synchronous motor such as shown in FIG.1, but showing a known construction;

FIGS. 3, 3A and 3B are sectional views showing a known method ofassembling a permanent magnetic rotor assembly for the motor shown inFIG. 2;

FIG. 4 is a sectional view showing a sub-assembly of parts for a rotorassembly under the present invention;

FIG. 5 is an end view of the sub-assembly taken along line 5--5 in FIG.4;

FIGS. 6 and 7 are sectional views showing the method of assembly underthe present invention; and

FIG. 8 is an end view of the complete rotor assembly, taken along line8--8 in FIG. 7.

The subject matter of the present invention may be incorporated in theconstruction of a synchronous motor 15, FIG. 1, typical of asubfractional motor having a cylindrical shaft-supported rotor.

The subject matter of the present invention is best understood byreferring to typical prior art. For this purpose FIG. 2 is presented toshow a known construction inside a motor of the type shown in FIG. 1.The housing 16 of the motor retains two windings or field coils 17surrounding a rotor 20. The rotor may be a permanent magnet of thebarium ferrite type. The rotor 20 is mounted on a shaft 21 constitutingthe mechanical out-put of the motor in the known construction, by a hub22. The hub 22 has an opening in the center through which the shaft isinserted, and cemented in place. The periphery of the hub is continuousand is bonded to the rotor as hereinafter explained.

One end 21E of the shaft extends beyond the hub 22, to serve as thethrust end of the shaft, being opposed by a leaf spring 24 to dampenmovement of the rotor assembly when the motor is energized.

The shaft 21 is rotatably supported by a single bearing 25 retained by abracket 26 secured to the housing 16.

Concentricity between the outer diameter (O.D.) of the rotor 20 and theaxis X of shaft 21, with some minimum tolerance, is important;otherwise, the rotor assembly will wobble, resulting in noise,inefficient operation and shortened bearing life.

One common method of attaining concentricity is shown in FIGS. 3, 3A and3B. A flowable plastic or resin 22A, such as an expoxy resin, is pouredor injected into the rotor, FIG. 3. It bonds naturally at the peripheryto the rotor after the resin material is hardened the collar or hub 22Athus afforded is machined in the center, FIG. 3A, to afford an opening22B for the shaft 21. The shaft 21 is then inserted into the opening 22Band is fixed thereto as by a suitable cement. A fixture, not shown, isemployed to assure concentricity within a minimum tolerance duringseating of shaft 21.

Variations are employed insofar as concerns providing an opening in thehub for seating the shaft. For example, it has been proposed to press analuminum washer into the cast resin (epoxy) before it sets, whereafterthe aluminum washer is machined to form an opening into which a knurledend of the shaft is pressed.

It will be recognized from the foregoing that several assembly steps arerequired incidental to coupling the shaft to the rotor by means of a hubin order to obtain concentricity in the critical dimension. Under thepresent invention, the steps are reduced in number at the same time ahub of less mass is attained, and it is unnecessary to extend the end ofthe shaft in order to achieve a thrust element.

In accordance with the present invention, FIGS. 4 and 5, a pre-moldedrotor hub 35 of a relatively deformable plastic, having an integralthrust protuberance 36 on one side is bonded to one end of a rotor shaft37. The hub 35 may be a body of relatively hard but nonethelessdeformable plastic such as Delrin, polypropylene or the like, but softerthan permanent magnet materials employed for magnetic rotors. The hub ordisc 35 is molded with a plurality of equidistantly spaced teeth 38presenting an outside diameter D which is slightly larger than theinside diameter D1 of a cylindrical permanent magnet rotor 40, FIG. 6,into which the hub is to be press fitted.

The assembly method is greatly simplified. First, the rotor 40 ispositioned within a recess 44 in a fixture 42, FIG. 6, having anaccurately centered bore 41, co-axial with the recess 44, for receivingthe shaft 37 to which the hub 35 is fixed. The shaft 37 is presented tothe bore 41 in the manner shown in FIG. 6. A force F is then applied ofsufficient magnitude to press the hub into the opposed end of the rotor40. Because of the fixturing, the shaft 37 will be concentric to theoutside diameter of the rotor 40 (radius r, FIG. 6). This concentricityis maintained during press fitting of the hub 35 even though there maybe some eccentricity between the inside diameter of the rotor and thetoothed periphery of the hub 35. A slight eccentricity is shown in FIG.6, but inasmuch as the teeth of the disc are shearable, being ofmaterial softer than the rotor, the hub teeth will differentiallydistort or bend, and will actually be shaved or trimmed, in the courseof seating the hub by the press fit as the shaft is being progressivelyextended into the bore 41. The press fit automatically locks the shaftto the rotor.

As will be evident from FIG. 8, the spaces 38S between the teeth 38reduce the mass of the hub, reducing inertia in the first instance.Vibration is reduced (if there is some eccentricity within a toleranceallowance) compared to the known structure where the periphery of thehub is continuous.

At the same time, the thrust protuberance 36 enables the shaft to beshortened in those instances where (see FIG. 2) the requirement is thatprovision be made for absorbing thrust. Further, thrust loading isapplied to the hub and is not applied directly to the end of the shaft,that is, the forces are absorbed by the plastic hub meaning a more quietmotor.

I claim:
 1. A method of assembling a cylindrical rotor and a metal rotorshaft into a rotor assembly for a subfractional dynamoelectric machinecomprising the following steps:A. presenting to one end of the rotor acircular hub molded substantially concentrically to one end of theshaft, said hub having a toothed periphery of deformable and shearableplastic material softer than the rotor, and the outer diameter of thehub being larger than the inner diameter of the rotor; B. press-fittingthe hub into the inner diameter of the rotor, while maintaining theouter diameters of the shaft and the rotor in concentric relation, theteeth of the hub distorting during press fitting to compensate for anyeccentricity between the peripheral ends of the teeth and the innerdiameter of the rotor.